This project will rigorously document the link between climate and the monthly variation of conceptions and births in the United States. This link will be tested for nonwhite fertility as well as white fertility, using a time-varying parameter model. Further testing of the link between climate modification (air conditioning) and the reduction of seasonal fluctuations in births will also be undertaken, using as the dependent variable parameters estimated from the time-varying parameter model, and as the explanatory variables, data on social and economic changes, in addition to the adoption of air conditioning, for each state of the U.S. Unexplained variation in seasonal patterns of births will be correlated with temperature extremes to test the climate-fertility link further and to provide for an extrapolation to the potential effects of climatic change on the level of fertility in the U.S.